When burning fossil fuels to produce energy, one typically uses a high temperature combustion process in the presence of air. Unfortunately, this type of process produces an gas effluent which comprises gaseous by-products of combustion including nitrogen oxides (NOx), carbon monoxide, and acid compounds such as sulfur dioxide (“SO2”), sulfur trioxide (“SO3”), hydrochloric acid (“HCl”), hydrofluoric acid (“HF”), The gas effluent may also contain unburned hydrocarbons and volatile heavy metals which originate from the fossil fuels.
Most of the NOx formed during the combustion process is the result of two oxidation mechanisms: (1) reaction of nitrogen in the combustion air with excess oxygen at elevated temperatures, referred to as thermal NOx; and (2) oxidation of nitrogen that is chemically bound in the coal, referred to as fuel NOx.
Nitrogen oxides (NOx) are well-known pollutants, and sulfur oxides (SOx) are harmful to health or the environment. Due to their significant impacts on the environment and health, there has been a growing environmental concern on SOx, NOx emissions to the atmosphere in the past two decades thus the importance to remove these materials from the combustion gas effluents prior to their release into the environment. Flue gas at coal-fired power stations is one of the main sources for the emissions of SOx and NOx. More and more stringent regulations on the emission of these pollutants come in force, which have put a high pressure on coal-fired power generators.
There have been many investigations into methods that allow for the removal of these substances. The methods developed and used are generally designed to address the removal of one type of pollutants.
Reducing NOx emissions is generally achieved by controlling the formation of NOx through combustion controls and/or by removing NOx after being formed through post-combustion controls. Adsorption techniques can be used for SOx removal. But combustion modifications and post-combustion control technologies may suffer from limited maximum removal of pollutants and limited capacity.
A method for addressing the problem of noxious waste gases is catalytic removal, which by comparison, is extremely effective in removing large proportions of pollutants and is capable of treating very large volumes of waste gases for long periods of time.
Selective catalytic reduction (SCR) has been one of the most effective technologies for removing NO (the main nitrogen oxide formed at high temperature) from a waste gas effluent originating from a stationary or mobile combustion source. Stationary combustion sources are mainly utility boilers, industrial boilers, incinerators, and cogeneration turbines. Mobile combustion sources are mainly vehicles such as automobiles, trucks.
The SCR process is widely used for example in the U.S., Japan, and Europe to reduce emissions of large utility boilers and other commercial applications. Increasingly, SCR processes are being used to reduce emissions in mobile applications such as in large diesel engines like those found on ships, diesel locomotives, automobiles and the like.
In order to effect the reduction of NOx in waste combustion gases through catalytic reduction processes, it is necessary either to introduce a reducing agent, such as ammonia, and/or to use the unburned hydrocarbons present in the waste gas effluent. The SCR process generally provides the reduction of NOx, (NO, N2O and NO2) species using the reducing agent (e.g., ammonia) in the presence of oxygen and a catalyst to produce molecular nitrogen and water.
Selective catalytic reduction (SCR) of NO by NH3 as reducing agent in the presence of O2, that causes the formation of NO2 (from the parent NO) in the gas reagent mixture, is a complex reaction can be schematized with the following series of main reactions (I) and (II):4NO+4NH3+O2→4N2+6H2O  (I)2NO2+4NH3+O2→3N2+6H2O  (II)
Other common reactions that could be taken into account when low O2 concentration is present in the gas mixture are the following reactions (III) and (IV):NO+NO2+2NH3→2N2+3H2O  (III)6NO2+8NH3→7N2+12H2O  (IV).
At temperatures around 300-350° C., competition between the reaction of NO reduction by NH3 and the reaction of NH3 oxidation by oxygen forming NO and/or N2 and/or N2O can occur, according to the following reactions (V) to (VII):4NH3+5O2→4NO+6H2O  (V)2NH3+2O2→N2O+3H2O  (VI)4NH3+3O2→2N2+6H2O  (VII).
These last reactions (V) to (VII) sequestrate NH3, the reducing agent, to the SCR process: they are in competition with the previous reactions of NOx reduction. The occurrence of these reactions (V) to (VII) is responsible for the observation of a maximum of the conversion curve of the NOx species as a function of reaction temperature; that is to say, the rate of NOx (NO+NO2) reduction is not continuously increasing with temperature because, staring from a defined temperature, the concentration of the reducing agent (NH3) decreases due to its oxidation by oxygen.
The SCR process is a competitive reaction scheme constituted by series of parallel reactions, the kinetics of each reaction and its dependence on temperature has to be exploited in order to selectively obtain NO reduction by ammonia and to avoid ammonia oxidation by oxygen. Finding a catalytic system to selectively obtain NO reduction by ammonia in a large temperature interval is thus challenging.
Various catalysts have been used in the SCR processes. Catalysts, including various metals, transition metal oxides, and mixed metal oxides have been employed for NO reduction. Initial catalysts, which employed platinum or platinum group metals, were found unsatisfactory because of the need to operate in a temperature range in which explosive ammonium nitrate forms. In response to environmental regulations in Japan, the first vanadium/titanium SCR catalyst was developed, which has proven to be highly successful. Further development has resulted in the development of vanadium catalyst deposited on titanium oxide/tungsten oxide support material.
Metal-promoted zeolite catalysts including, among others, iron-promoted and copper-promoted zeolite catalysts, for the selective catalytic reduction of nitrogen oxides with ammonia are known.
Zeolites are aluminosilicate crystalline materials having rather uniform pore sizes which, depending upon the type of zeolite and the type and amount of cations included in the zeolite lattice, range from about 3 to 10 Angstroms in diameter.
Chabazite (CHA) is a small pore zeolite with 8 member-ring pore openings (“3.8 Angstroms) accessible through its 3-dimensional porosity. A cage like structure results from the connection of double six-ring building units by 4 rings. WO 2008/106519 discloses a catalyst comprising: a zeolite having the CHA crystal structure and a mole ratio of silica to alumina greater than 15 and an atomic ratio of copper to aluminum exceeding 0.25. The catalyst is prepared via copper exchanging NH4+-form CHA with coppersulfate or copperacetate. The copper concentration of the aqueous copper sulfate ion-exchange step varies from 0.025 to 1 molar, where multiple copper ion-exchange steps are needed to attain target copper loadings. U.S. Pat. No. 8,293,199 describes processes for the preparation of copper containing molecular sieves with the CHA structure having a silica to alumina mole ratio greater than about 10, wherein the copper exchange step is conducted via wet state exchange and prior to the coating step and wherein in the copper exchange step a liquid copper solution is used wherein the concentration of copper is in the range of about 0.001 to about 0.25 molar using copperacetate and/or an ammoniacal solution of copper ions as copper source.
Acidic zeolites such as copper ion-exchanged Y zeolites, H mordenite, and Cu—H mordenite have been reported to be efficient catalysts for NO reduction by NH3, particularly for the high-temperature application of SCR technology—see for example, Choi et al, Journal of Catalysis (1996) vol. 161, pages 597-604; Putluru et al, Applied Catal. B: Environmental (2011) vol. 101, pages 183-188.
Iron-promoted zeolite beta (U.S. Pat. No. 4,961,917) has been an effective commercial catalyst for the selective reduction of nitrogen oxides with ammonia.
Unfortunately, it has been found that under harsh hydrothermal conditions, for example exhibited during regeneration of a soot filter with temperatures locally exceeding 700° C., the activity of many metal-promoted zeolites begins to decline. This decline is often attributed to dealumination of the zeolite and the consequent loss of metal-containing active centers within the zeolite.
The catalysts employed in the SCR process ideally should be able to retain good catalytic activity over the wide range of temperature conditions of use, for example, from 200° C. to 600° C. or higher, preferably under hydrothermal conditions since water is generated during NOx reduction (see reactions (I) and (II) above). Hydrothermal conditions are also encountered in practice, such as during the regeneration of a soot filter, a component of the exhaust gas treatment system used for the removal of particles.
There are two basic options for controlling SOx emissions from coal-fired power plants, which is formed from the oxidation of sulfur in the fuel: (1) switching to lower sulfur fuels; and (2) SOx capture, including Flue Gas Desulfurization (FGD), or more commonly referred to as “scrubbing.”
Traditional flue gas desulfurizer (FGD) systems are built for the express purpose of removing SOx from the exhaust flue gases of fossil-fuel power plants and sometimes from the emissions of other SOx emitting industrial processes. These systems generally employ five removal methods:                wet scrubbing that uses alkaline sorbent or seawater to scrub the flue gas;        the spray-dry scrubbing that uses similar sorbent slurries;        wet sulfuric acid process that recovers sulfur in the form of sulfuric acid;        SNOX flue gas desulfurization that removes sulfur dioxide, nitrogen oxides and particulates from flue gases; and        dry sorbent injection systems        
FGDs employ two stages: one to remove fly ash and the other to remove SOx. In wet scrubbing systems, the flue gas passes through a fly ash removal device, either an electrostatic precipitator or a wet scrubber, and then into the SOx-absorber. In dry injection or spray drying operations, the SOx reacted first with the sorbent, and then the flue gas passes through a particulate control device.
Cost-effective and sustainable technologies for the reduction of such pollutants (emissions targeted include NOx, SOx, mercury and other heavy metals, halogens and particulate matter) from flue gas have become increasingly important nowadays. With multi-pollutant emissions control technologies, a single system should be able to remove multiple pollutants from flue gas before they are released into the atmosphere. A considerable advantage of a multi-pollutant strategy is lower capital investment when compared with investing in several different technologies to address each pollutant. Likewise, the installation of a single unit is faster and requires less downtime. This is especially relevant due quickly approaching deadlines. Also, multi-pollutant technologies generally require a smaller footprint since their processes are encapsulated in one system.
Despite the advantages of multi-pollutant emissions control technologies, there is still a barrier to larger scale deployment in the power sector because there is a level of comfort in using the traditional emissions control technologies because they have been used by the industry for years. The owners of fossil-fired power plants know the capital investment, and operations and maintenance, costs of older technologies and operators can more easily anticipate problems that may arise.
Even though numerous attempts have been made aiming at developing technologies for the removal of SOx and NOx, not much effort has been made on the simultaneous conversion of NOx and SOx in flue gas, especially for once-through applications in which treatment material is injected into the waste gas stream and where the contact time between the injected treatment material and the contaminated gas containing these Sox, NOx pollutants may be short in the order of a second to a few seconds.